如何在 SwiftUI 中声明 viewModel 对象？

Question

I am trying to use an @ObservableObject viewModel which is declared as @ObservedObject inside my view struct. The problem is that when the viewModel changes it's "domains" property @Published var, the UI is not updating. Also, I change domains inside getDomains() function which is called inside init() {}. Looks like it is called twice, why is that happening? Here's my code for viewModel:

import Combine

class DomainsViewModel: ObservableObject {

    @Published var domains: [InterestDomain] = [InterestDomain]()
    
    init() {
        self.getDomains { (response) in
            print(response)
        }
    }
    
    func getDomains(completion: @escaping (Bool) -> Void) {
        NetworkEngine.shared.appNetwork.getInterestDomains { result in
            self.domains.removeAll()
            switch result {
            case .success(let domainsOfInterest):
                if let domainsList = domainsOfInterest {
                    self.domains = domainsList
                }
                completion(true)
            case .failure(_):
                completion(false)
            }
        }
    }

}

Code for the view: import Foundation import SwiftUI import Combine

struct DomainsOfInterestView: View {

    @ObservedObject var viewModel: DomainsViewModel = DomainsViewModel()
    
    @State var isActive = true
    
    var body: some View {
            VStack(alignment: .center) {
                HStack {
                    Text("Choose domains of interest for your profile")
                        .font(.headline)
                    Spacer()
                }.padding(.bottom, 16)
                
                ForEach(viewModel.domains.indices) { index in
                    DomainOfInterestElement(isActive: self.$isActive, domain: self.viewModel.domains[index].name)
                }

                OrangeButton(action: {
                }) {
                    Text("Save")
                }.padding(.top, 30)
                    .padding([.leading, .trailing], 30)
                Spacer()
            }.padding([.leading, .trailing], 12)
            .navigationBarTitle("Domains of interest")
    }
}

struct DomainsOfInterestView_Previews: PreviewProvider {
    static var previews: some View {
    DomainsOfInterestView()
    }
}

struct DomainOfInterestElement: View {
    @Binding var isActive: Bool
    var domain: String
    var body: some View {
        Button(action: {
            self.isActive.toggle()
        }) {
            VStack {
                Divider().padding(.bottom, 12)
                HStack {
                    checkBoxView()
                        .frame(width: 36, height: 36)
                    textView().font(.custom("SFProDisplay-Regular", size: 16))
                    Spacer()
                }
            }
        }
    }
    
    func checkBoxView() -> Image {
        switch isActive {
        case true:
            return Image(uiImage: #imageLiteral(resourceName: "check-box-active-2-1")).renderingMode(.original)
        case false:
            return Image(uiImage: #imageLiteral(resourceName: "check-box-active-1")).renderingMode(.original)
        }
    }
    func textView() -> Text {
        switch isActive {
        case true:
            return Text(domain)
                .foregroundColor(.black)
        case false:
            return Text(domain)
                .foregroundColor(Color.orGrayColor)
        }
    }
}

Could anyone help me please? Thanks.

Answer 1

DomainsOfInterestView is recreated on every update of its view model. And you initialise the view model property with a new DomainsViewModel instance every time. A new view model will have the domains property set to [InterestDomain]() and self.getDomains will be called again.

First of all, better to inject the viewModel to DomainsOfInterestView in an initiailiser as @youjin wrote in a comment to the previous post. Also, you can use @StateObject instead of @ObservedObject if your minimum deployment target allows that. In this case, the viewModel property won't be reset every time the view is updated.

Answer 2

Quoting the answer from here

If your data can be changed, you need a dynamic ForEach loop (with an explicit id parameter):

 ForEach(viewModel.domains.indices, id: \.self) { index in // <=
    // ...
 }

In some cases Xcode warns you when you try to modify your array used in a ForEach loop:

ForEach( :content:) should only be used for constant data. Instead conform data to Identifiable or use ForEach( :id:content:) and provide an explicit id!

Tested code

struct DomainsOfInterestView: View {

    @ObservedObject var viewModel: DomainsViewModel = DomainsViewModel()
    
    @State var isActive = true
    
    var body: some View {
            VStack(alignment: .center) {
                HStack {
                    Text("Choose domains of interest for your profile")
                        .font(.headline)
                    Spacer()
                }.padding(.bottom, 16)
                
                ForEach(viewModel.domains.indices, id: \.self) { index in // <=
                    Text(String(index))
                }

                Button(action: {
                }) {
                    Text("Save")
                }.padding(.top, 30)
                    .padding([.leading, .trailing], 30)
                Spacer()
            }.padding([.leading, .trailing], 12)
            .navigationBarTitle("Domains of interest")
    }
}


class DomainsViewModel: ObservableObject {

    @Published var domains: [Int] = [Int]()
    
    init() {
        self.getDomains { (response) in
            print(response)
        }
    }
    
    func getDomains(completion: @escaping (Bool) -> Void) {
        DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
            self.domains = [1,2,3,4,5]
            completion(true)
        }
    }

}

Tested with Xcode 12 beta 6 on iOS 14 beta 6